Back rest for a chair



FIG. 1 is a perspective view showing the front and left side of a back rest for a chair which forms my new design, the broken lines in this and other figures showing other parts of a chair being for illustrative purposes only and forming no part of my design;

FIG. 2 is a front elevational view thereof; and,

FIG. 3 is a left side elevational view thereof, this view and the opposite right side elevational view being mirror images. 

The ornamental design for a back rest for a chair, as shown and described. 